To date, neuroblastoma, melanoma, Myc sellekchem driven lymphomas, leukemia and lymphoma cell lines and Fanconis Anemia cells have been identified to be Chk1 kinase dependent. This suggests that DNA repair, DNA damage response or DNA replication may be suitable candidates for single agent Chk1 inhibi tor therapy. Spontaneous triple negative breast cancer shares many of the characteristics of tumors derived from patients carrying mutations in the BRCA gene. BRCA is known to be involved in a variety of DNA repair pathways such as homologous recombination and base excision repair and are exquisitely sensitive to inhibitors of poly polymerase. Recent reports have demons trated the sensitivity of TNBC to PARP inhibitors as well as DNA damaging cytotoxic agents such as gemcitabine and cisplatin.

TNBCs have been shown to have reduced expression of DNA repair genes involved in base excision repair, nucleotide excision repair and the Fanconis Anemia repair pathways. This suggests that like cancers that arise in BRCA mutation carriers, spontaneous TNBCs may harbor underlying defects in DNA repair and we hypothesized that this cancer sub type may be a suitable candidate for single agent Chk1 inhibitor therapy. This hypothesis was substantiated in a screen of 26 solid cancer cell lines where TNBC cancer cell lines were among the most sensitive to growth inhibition by the novel, selective Chk1 inhibitor V158411. Even though V158411 is an extremely selective inhibitor of Chk1, it is difficult to ascertain the absolute selectivity of any small molecule kinase inhibitor.

To confirm this observation, two structurally unrelated Chk1 kinase inhibitors PF 477736 and AZD7762 potently inhibited the proliferation of TNBC breast cancer cell lines compared to two ER positive cell lines suggesting that the anti proliferative effects observed were due to Chk1 inhi bition and not the inhibition of an off target kinase. However, the sensitivity to the Chk1 inhibitors was not just limited to the TNBC cell lines as the HER2 postive, ER negative SKBr3 breast cancer cell line and the SKOV 3 ovarian cancer cell line were among the most sensitive cell lines to Chk1 inhibitor induced cell death. These results have recently been confirmed by another study demonstrating that four TNBC cell lines had reduced viability following Chk1 knockdown with a specific siRNA.

Additional studies have demonstrated that the Chk1 inhibitor AZD7762 synergistically combined with numerous PARP1 inhibitors to inhibit the growth of mammary carcinoma cells in vitro and in vivo. In these studies, AZD7762 demonstrated little sin gle agent activity in the breast cancer cell lines at the V158411 GSK-3 3. 1 1. 4 concentration tested. Of the cell lines tested in these pa pers, only two overlapped with our study. In light of these results, we attempted to understand the mechanism by which single agent Chk1 inhibitors induced TNBC and ovarian cancer cell death.